Current water demands have prompted the investigation of alternative water sources and ways to augment current water supplies. It has been said that, “Nothing in the future will have a greater impact on our ability to sustain our way of life and preserve our environment for future generations than water.” (The Statewide Water Supply Initiative, Colorado Department of Natural Resources.). These concerns transcend Colorado and the Western United States and apply to the world resource outlook in general.
One potential source of augmentation water is the water included in hydrocarbons extracted from geological formations containing oil and natural gas. The water included with the oil and/or gas produced from the well is termed “produced water” or “production water.” Prior to this invention, production water had not been considered a potential source of augmentation water. Indeed, it was a difficult and expensive task just to make production water suitable for disposal.
Typically production water is separated from the hydrocarbons using an “API” oil water separator. The principle of the API separator is to allow for the non-aqueous phase liquids (primarily the organics which are lighter than water) to float to the surface. Then the organics are removed from the production water and concentrated through the use of a heat treatment unit, which drives off the remaining water through evaporation.
The API separator will recover the majority of the oil, but dissolved materials and volatile organics will remain in the aqueous segment. Thus, production water usually contains high concentrations of hydrocarbons and other inorganic constituents. Typically production water is disposed of by being re-injected under pressure back into the geologic formation, through a Class II injection well, permitted by the US EPA. Because of the contaminants in the production water, injection into other geological formations that can be used for a drinking water source or into surface water is usually prohibited. In addition, re-injection is costly because it requires substantial pressure (and, therefore energy) to overcome the resistance within the geological formation. The Department of Energy estimates that 30 to 40 percent of the energy obtained from the formation as oil is used to re-inject or move this water. (DOE—Sandia Conference, Salt Lake City, January 2006.) In addition, re-injection of production water into the formation dilutes subsequently-produced oil, adding additional costs to the recovery and processing of those hydrocarbons. Nevertheless, prior to the present invention, re-injection was the most straightforward method to dispose of production water, since it was quite difficult and costly to clean the production water sufficiently for direct discharge. “Direct discharge” is a term of art connoting discharge directly through a pipe to the surface water course or stream.
Thus, an efficient and effective treatment for upgrading production water would be beneficial both in providing high-quality water that can be used in various water conservation schemes and in avoiding the costs and other detriments of re-injecting the production water under ground.
As used herein “production water” means water separated from the production stream of oil and gas wells. An example of the constituents in a sample of production water from Wellington, Colo.—after API separation—is shown in Table 1
TABLE 1Produced Water Quality Parameters After the Oil/Water SeparationTypical Range of ProcessValues mg/lInorganicsTotal Dissolved Solids (TDS)12006000Total Hardness as CaCO330300Total Alkalinity as CaCO310004000Chloride (Cl)401000Fluoride<110Phosphate (PO4)<0.530Nitrite+Nitrate-Nitrogen <0.540(NO2+NO3-N)*MetalsAntimony (Sb)<0.0051.00Arsenic (As)*<0.0051.00Barium (Ba)*3.0030.00Berylium (Be)<0.00051.00Boron (B)1.0010.00Cadmium (Cd)<0.0011.00Chromium (Cr)<0.021.00Copper (Cu)<0.011.00Iron (Fe)*0.1030.00Lead (Pb)<0.0055.00Manganese (Mn)*<0.00510.00Mercury (Hg)<0.00020.10Nickel (Ni)*<0.0510.00Selenium (Se)<0.0055.00Silver (Ag)<0.015.00Thallium (Tl)*<0.0021.00Zinc (Zn)<0.00510.00OrganicsOil and grease*20.0200.00Benzene*1.0010.00Toluene*1.005.00Ethylbenzene*0.101.00Xylenes, total*1.005.00n-Butylbenzene*0.010.50sec-Butylbenzene*0.010.10tert-Butylbenzene*0.01 0.10Isopropylbenzene*0.01 0.104-Isopropyltoluene*0.01 0.10Naphthalene*0.01 0.10n-Propylbenzene*0.01 0.101,2,4-Trimethylbenzene*0.10 1.001,3,5-Trimethylbenzene*0.10 1.00Bromoform*<0.0011.00This production water also contains paraffins and asphaltenes in an unmeasured, but not insignificant, amount.
Production water contains both inorganic and organic constituents that limit the discharge options available to the producer. Produced water contains a range of constituents including dispersed oil, dissolved or soluble organics, produced solids, scales (e.g., precipitated solids, gypsum (CaSO4), barite (BaSO4)), bacteria, metals, low pH, sulfates, naturally occurring radioactive materials (NORM), and chemicals added during extraction (Veil, et al., 2004). The oil related compounds include benzene, xylene, ethyl benzene, toluene, and other compounds of the type identified in the sample analysis shown in Table 1 and in other crude oil and natural gas sources. Normally, the production water will also contain metals, e.g., arsenic, barium, iron, sodium and other multivalent ions, which appear in many geological formations.
In order to produce a higher grade of water, for example, either “agricultural” or “augmentation” water, both the hydrocarbon components and heavy metals need to be removed. As used herein, “agricultural water” means water that will meet the basic standards dictated by the EPA or state agency as the primary agency for water quality in surface waters. “Potable water” means water that meets the primary and secondary drinking water standards as defined by 40 CFR Sec.136.
As used herein “augmentation water” means water that can be used to augment a water source, i.e., agricultural, industrial, municipal, irrigation or potable water. In a more restrictive sense it also means water that is supplied to keep a stream whole. In the nomenclature used for water rights in the Western portion of the United States “augmentation water” means water that protects individuals or water users that have a prior appropriation for the use of that water. A water augmentation plan is a procedure for replacing water to a stream system whose flows are depleted by the consumption of water, where the water user does not have a right to the water consumed. Consumption or “consumptive use” means the water has been placed in the evapo-transpiration cycle or otherwise not returned to the stream system. According to current ground water laws in the west with prior appropriation, if water under the land would reach a stream system within approximately 100 years, it is deemed to be “tributary” to that stream system; it supports the stream's flow. Other users may have rights to the stream flow; therefore, a new user cannot consume the water unless the new user has a “water right” (decreed by a Water Court or by a State Engineer) which allows their use of the water. Otherwise, a downstream user with senior water rights could be damaged because he might not have enough water for his purpose. So, absent a water right, the new user must figure out a way to replace or “augment” his water use so the existing stream flow remains the same as before he used it. Augmentation may be made by purchasing water rights on the affected stream system or by physically replacing the water used from another legal water source. An augmentation plan is submitted to the Water Court or State Engineer which governs the particular drainage basin in which the affected stream system lies. If the Court or State Engineer approves the plan, it will issue a decree which grants the use of the “tributary” water, provided that ongoing augmentation (replacement of used water) of that use occurs per the plan that is used by junior appropriators to obtain water supplies through terms and conditions approved by a water court that protect senior water rights from the depletions caused by the new diversions, under the Prior Appropriation Doctrine. Typically this will involve storing junior water when in priority and releasing that water when a call comes on; purchasing stored waters from federal entities or others to release when a river call comes on; or purchasing senior irrigation water rights and changing the use of those rights to off-set the new user's injury to the stream. These plans can be very complex and it is suggested that an engineering consultant be retained to allow for proper consideration of all hydrologic and water right factors.
Prior art methods of cleaning and upgrading production water have been ineffective and/or overly expensive. These methods include:                Oil Water Separation (API method): The normal method for oil water separation is the use of an API oil water separator. The principal of the API separator is to allow for the non-aqueous phase liquids (“NAPL's”) to float to the surface. Then the organics or NAPL's are removed from the production water and concentrated through the use of a heat treatment unit. The oil water separator will recover a majority of the oils, but any dissolved materials in the remaining production water will not be removed by the API unit. Thus, the method is useful in recovering incremental amounts of oil from the production water, but is ineffective in removing other contaminants from the production water.        Precipitation: Precipitation is used for the removal of both dissolved oils and heavy metals. The precipitation will react with the dissolved oil and then flocculate and precipitate the oil into a particle. This particle can then be removed through floatation and filtration, i.e., the coagulant entraps both the metal and oil particles and makes them “bigger” so they can either float or be filtered from the solution. In some instances, it has been suggested to further clean the effluent from the precipitation stage by reverse osmosis. However, precipitation and filtration is still ineffective in removing volatile organic compounds, such as benzene. Further, processing would be required to remove those organic compounds.        Adsorption: Activated carbon adsorption has been used for many years as a method for the removal of dissolved organics. Activated carbon will remove organics typically below method detection limits listed in 40 CFR 136. However, this technology is very expensive, and it does not normally remove heavy metals.        Nano Filtration: Nano filtration has been used for the removal of sulfate ions in the field and has been shown to be very effective. However, this would require microfiltration and activated carbon for organic removal.        Organo-thiol ligands: The use of organo-thiol ligands has proved very promising in the removal of specific toxic heavy metals and dissolved organics from wastewater. However, they are very expensive and work on a limited number of metal ions.        SMZ Removal—Application of “surfactant modified zeolites” is also a technique utilized on produced waters for the removal of benzene, toluene, ethylbenzene, and xylene, i.e., “BTEX,” and other volatile organics. The technique is most effective on benzene but is also effective on other organics. This technology does not remove heavy metals, unless they are associated with the organics being removed.        
These prior art processes are all limited to certain aspects of cleaning up production water and do not present a comprehensive solution for upgrading production water to agricultural grade or potable water. Methods that have attempted to achieve that result comprise expensive multiple step processes that sequentially and separately attempt to address each problem in cleaning up production water. Thus, for example, one process of cleaning up production water included separate steps for: warm softening; coconut shell filtration; cooling (fin-fan); trickling filtration; pressure filtration; ion-exchange; and reverse osmosis. (R. Funston et al., “Evaluation of Technical and Economic Feasibility of Treating Oilfield Produced Water to Create a ‘New’ Water Resource,” (Ground Water Production Council Conference, Produced Waters Workshop, Colorado Springs, Colo., October 2002.)
Obviously, there is a need for a simple, economic process to produce higher grade water such as agricultural and/or potable water, from oil and gas production water.
Although the following description and example are focused on production water from oil and gas wells, it is anticipated that the invention may also have applicability to production water from gas wells, and other similar water-containing hydrocarbon materials, such as coal bed methane water, obtained from geological formations.